The erection of the memorial both on the river and on the land, might be intended to show the salvation of Jesus both by land and by water. Psalms 77:16-20.
Related Commentaries of Joshua 4:9
Joshua 4:9-10
9 And Joshua set up twelve stones in the midst of Jordan, in the place where the feet of the priests which bare the ark of the covenant stood: and they are there unto this day.
10 For the priests which bare the ark stood in the midst of Jordan, until every thing was finished that the LORD commanded Joshua to speak unto the people, according to all that Moses commanded Joshua: and the people hasted and passed over.